The present invention concerns the fabrication of integrated circuits and pertains particularly to the wet etch removal of underlayer material after performing chemical mechanical polishing on a primary layer.
The trend in integrated circuit manufacturing is to increase the amount of circuitry within each individual integrated circuit. Along with this trend there is an increase in the number of layers used and a decrease in the dimensions of the patterned layers.
As the pattern dimensions within integrated circuits shrink in size, it is necessary to provide more accurate methods to accurately and uniformly perform the processing steps.
When processing integrated circuits, masks are utilized to pattern and etch layers of material deposited on a wafer. These layers are composed of, for example, polysilicon, metal or oxide.
As optical resolution increases, the depth of focus of the mask image correspondingly narrows. This is due to the narrow range in depth of focus imposed by the high numerical aperture lenses in photolithography tools which use the mask images. This narrowing depth of focus is often the limiting factor in the degree of resolution obtainable, and thus, the smallest components obtainable using the photolithography. Unevenness in the surface of a wafer exaggerate the effects of decreasing depth of focus. Thus, in order to properly focus the mask image defining sub-micron geometries, a precisely flat (i.e. fully planarized) surface is desired. A fully planarized surface will allow for a small depth of focus, thus aiding in the definition and subsequent fabrication of extremely small components.
Chemical-mechanical polishing (CMP) is generally used to aid in planarization of a wafer. When performing CMP, a sacrificial layer of dielectric material is removed using mechanical contact between a wafer and a moving polishing pad saturated with slurry. Polishing flattens out height differences, and thus planarizes the topography of a wafer.
CMP is widely accepted as the preferred process for planarizing dielectric materials and metal plugs placed on a wafer. This is particularly true when the technology uses dimensions on the order of 0.25 micrometers or smaller. Slurry used in such a CMP process generally includes two parts. The first part is a pH-controlled chemical solution. The second part is an abrasive.
For example, during CMP-based dielectric planarization, the chemical solution may be an aqueous alkaline solution which depolymerizes the glass surface forming soft hydroxides. This chemical solution is used by the abrasive action of colloidal silica particles in the slurry. For additional information on this process, see L. Cook, Journal of Noncrystalline Solids, 120,152 (1990).
In metal CMP processes, slurries help to form soft abradable metal oxides which can easily be removed by the mechanical action of the abrasives in suspension. Slurries are typically targeted to remove one particular film and further optimized to remove other secondary films (exposed during CMP) at a certain removal rate (RR). A low removal rate is used if it is desired to stop on the film. A high removal rate is used if polishing of the film is desired.
However, given the many competing requirements for a particular slurry, it may not be feasible to meet the requirement of easily removing secondary films. In such cases, the polishing step is increased in time (which can significantly degrade throughput) or the layer is left behind and removed subsequently during metal etch (but this adds to the metal stack height and the complexity of metal etch). Either solution impacts wafer processing throughput and thus increases the cost of the process.